The symptoms are consistent with a hardware driver problem, which is not that surprising considering that even some of the Vista approved drivers don't work. Is this a clean install? Did you run the Vista Upgrade advisor?
You could try removing all unnecessary hardware during the boot, then add bits back one at a time until it falls over. Most of the reported problem seem to be to do with printers, scanners, cameras and video cards.
Also if you are upgrading and you have older anti virus products (or anything else that tries to mess with the kernel), they too can cause major problems.
Be aware that even if you get the machine working, you may well find that a lot of your existing software either won't work or will behave erratically.
Frankly, except that it's my job to evaluate it and support some of our (suicidal) customers who want to use it, I wouldn't touch Vista with a bargepole!